This invention relates to a semiconductor pressure detector (or sensor) apparatus, and more particularly to a semiconductor pressure detector apparatus having means to carry out zero-point temperature compensation.
In order to measure mass, stress, fluid pressure etc., various gauges have been used. Among them, a semiconductor strain gauge of high sensitivity exploiting the piezoresistive effect of a semiconductor has come into wide use in recent years.
The semiconductor strain gauge exploiting the piezo effect of a semiconductor has the advantage that the rate of change of the resistance to the strain or the gauge factor is high, whereas it has the disadvantage that the resistance value and gauge factor of the gauge exhibit great temperature-dependencies and are unstable.
In general, the resistance value R of the semiconductor strain gauge is given by the following expression: EQU R=R.sub.o (1+.alpha.T){1+S.gamma.(1+.beta.T)} (1)
where R.sub.o denotes the resistance value of the gauge in the strainless state at a predetermined temperature, T the temperature of the semiconductor strain gauge, S the strain, .alpha. the temperature coefficient of the resistance, .beta. the temperature coefficient of the gauge factor, and .gamma. the gauge factor. The gauge factor .gamma. has its value and polarity varied depending upon the orientation of a semiconductor single crystal, the angle defined by current and stress within the gauge, etc.
Expression (1) is expanded as follows: EQU R=R.sub.o (1+.alpha.T)+R.sub.o (1+.alpha.T)(1+.beta.T)S.gamma.(2) EQU .perspectiveto.R.sub.o (1+.alpha.T)+R.sub.o {1+(.alpha.+.beta.)T}S.gamma.(3)
The second term of the right hand of Expression (2) is the variation of the gauge resistance due to the strain. On the other hand, the coefficient .alpha. varies depending upon an impurity concentration within the crystal of the semiconductor strain gauge and has a value of 3000-600 ppm/.degree.C. in case of silicon single crystal by way of example, and the coefficient .beta. is independent of the impurity concentration and has a value of approximately -2000 ppm/.degree.C. in the case of silicon single crystal. The variation of the gauge resistance can have its temperature-dependency made low because, as apparent from the second term of Expression (3), the temperature coefficient .alpha. of the resistance of the semiconductor strain gauge and the temperature coefficient .beta. of the gauge factor can be cancelled by appropriately selecting the impurity concentration within the crystal. Accordingly, a strain-electric signal conversion bridge which employs the semiconductor strain gauges is often driven by a constant-current source in order to provide only the variation of the resistance as an output signal.
It has also been known that, in a strain-electric signal conversion apparatus of high precision, the drive current is varied depending upon the temperature in order to further reduce the temperature-dependency.
The output of the strain-electric signal conversion bridge at the time when the strain is zero exhibits the so-called temperature-dependency in which it changes with a temperature change, on account of the discrepancies of the resistance values R.sub.o and their temperature coefficients .alpha. of the plurality of gauges constituting the bridge. This temperature-dependency is the zero-point temperature-dependency, and it is the zero-point temperature compensation that reduces and compensates for such temperature-dependency.
For example, U.S. Pat. No. 3,654,545 entitled "SEMICONDUCTOR STRAIN GAUGE AMPLIFIER" (issued on Apr. 4, 1972) discloses a semiconductor strain gauge amplifier which includes a temperature sensor such as thermistor in order to realize such zero-point temperature compensation. Such compensation circuit, however, is complicated disadvantageously.
Other related references are:
U.S. Pat. No. 3,528,022 entitled "Temperature Compensating Networks" and
British Pat. No. 1,340,635 entitled "Improvements in Direct Current Pressure Ratio Circuit".